//C16.21:vslice.cpp -- using valarray slices
#include <iostream>
#include <valarray>
#include <cstdlib>

const int SIZE = 12;
typedef std::valarray<int> vint;    // simplify declarations
void show(const vint & v, int cols);
int main(void)
{
    using std::slice;   // from <valarray>
    using std::cout;
    vint valint(SIZE);    // think of as 4 rows of 3

    int i;
    for(i = 0; i < SIZE; i++)
    {
        valint[i] = std::rand() % 10;
    }

    cout << "Original array:\n";
    show(valint, 3);     // show in 3 columns
    vint vcol(valint[slice(1,4,3)]);    // extract 2nd column
    cout << "Second column:\n";
    show(vcol, 1);    // show in 1 column
    vint vrow(valint[slice(3,3,1)]);  // extract 2nd row
    cout << "Second row:\n";
    show(vrow, 3);
    valint[slice(2,4,3)] = 10;    // assign to 2nd column
    cout << "Set last column to 10:\n";
    show(valint, 3);
    cout << "Set first column to sum of next two:\n";
    // +not defined for slices, so convert to valarray<int>
    valint[slice(0,4,3)] = vint(valint[slice(1,4,3)]) +
                           vint(valint[slice(2,4,3)]);
    show(valint, 3);                           
}

void show(const vint & v, int cols)
{
    using std::cout;
    using std::endl;
    
    int lim = v.size();
    for(int i = 0; i < lim; i++)
    {
        cout.width(3);
        cout << v[i];
        if(i % cols == cols -1)
        {
            cout << endl;
        }
        else 
        {
            cout << ' ';
        }
    }
    if(lim % cols != 0)
    {
        cout << endl;
    }
}